1. Field of the Invention
The present invention is directed to an acoustic lens having variable focal length for utilization in an acoustic propagation medium.
2. Description Of the Prior Art
Acoustic lenses of the type having a variable focal length are employed, for example, in medical technology and serve for focusing acoustic waves in therapeutic as well as in diagnostic systems. The acoustic waves, for example, can be ultrasound waves or pressure pulses such as shockwaves.
German OS 37 39 393 discloses a lithotripter having an acoustic lens two lens walls, namely an entry wall and an exit wall, with at least one lens wall being deformable for varying the focal length. This lens functions as a positive lens that serves the purpose of focusing the pressure pulses generated with the lithotripter. It has been observed in this known acoustic lens that unintended variations in focal length as well as unintended changes in the geometry of the focus zone of the existing pressure, i.e. unintended changes of the acoustic properties of the lens, occur when the respective static pressures of the lens liquid and the acoustic propagation medium adjoining the deformable lens wall do not coincide. (The focus zone means that region surrounding the location of maximum pressure which lies within the -6 dB isobar.)